Referring to FIG. 1, an exemplary magnetic storage system 2 such as a hard disk drive is shown. A buffer 3 stores data that is associated with the control of the hard disk drive. The buffer 3 may employ SDRAM or other types of low latency memory. A processor 4 performs processing that is related to the operation of the hard disk drive. A hard disk controller (HDC) 6 communicates with the buffer 3, the processor 4, a host 7, a spindle/voice coil motor (VCM) driver 8, and/or a read/write channel circuit 10.
During a write operation, the read/write channel circuit (or read channel circuit) 10 encodes the data to be written onto the storage medium. The read/write channel circuit 10 processes the signal for reliability and may include, for example error correction coding (ECC), run length limited coding (RLL), and the like. During read operations, the read/write channel circuit 10 converts an analog output from the medium to a digital signal. The converted signal is then detected and decoded by known techniques to recover the data written on the hard disk drive.
One or more hard drive platters 11 include a magnetic coating that stores magnetic fields. The platters 11 are rotated by a spindle motor that is schematically shown at 12. Generally the spindle motor 12 rotates the hard drive platter 11 at a fixed speed during the read/write operations. One or more read/write arms 14 move relative to the platters 11 to read and/or write data to/from the hard drive platters 11. The spindle/VCM driver 8 controls the spindle motor 12, which rotates the platter 11. The spindle/VCM driver 8 also generates control signals that position the read/write arm 14, for example using a voice coil actuator, a stepper motor or any other suitable actuator.
A read/write device 15 is located near a distal end of the read/write arm 14. The read/write device 15 includes a write element such as an inductor that generates a magnetic field. The read/write device 15 also includes a read element (such as a magneto-resistive (MR) sensor) that senses the magnetic fields on the platter 11. A preamplifier (preamp) circuit 16 amplifies analog read/write signals. When reading data, the preamp circuit 16 amplifies low level signals from the read element and outputs the amplified signal to the read/write channel circuit 10. While writing data, a write current that flows through the write element of the read/write channel circuit 10 is switched to produce a magnetic field having a positive or negative polarity. The positive or negative polarity is stored by the hard drive platter 11 and is used to represent data.
Referring now to FIG. 2, the read channel circuit 10 outputs write signals wdx and wdy to the preamp circuit 16 when writing data. The preamp circuit 16 amplifies the write signals using a write amplifier 18. The amplified write signals are output to the read/write device 15. When reading data, the preamp circuit 16 receives signals from the read/write device 15, amplifies the signals using a read amplifier 19, and outputs amplified read signals rdx and rdy to the read channel circuit 10.
Some magnetic storage systems employ giant magneto-resistive (GMR) sensors as the read element. GMR sensors are more sensitive to magnetic transitions than MR sensors. For example, the GMR sensors are typically twice as sensitive as MR sensors. GMR sensors and other read elements are highly sensitive to electrostatic discharge (ESD). For example, the GMR sensor may have an ESD voltage tolerance of approximately 1V. GMR sensors are typically biased at 0.5V or lower during normal operating conditions. The risk of damage to a read element from ESD is greatest during manufacturing when the circuit is handled. Static discharge may occur when the circuit is handled which may damage the read element.
GMR sensors are typically protected from ESD damage by diode shunting circuits, which limit the maximum voltage that is applied to the GMR sensor. The maximum voltage is limited to a forward biased turn-on voltage of a single diode. Silicon junction diodes typically have a forward-biased turn on voltage between 0.7V and 0.8V. Schottky diodes typically have a forward-biased turn-on voltage between 0.4V and 0.5V.
Referring now to FIG. 3, the preamp circuit 16 includes an ESD protection circuit 30 that limits a maximum voltage that is applied to a read element 32 in the read/write device 15. The ESD protection circuit 30 includes first, second, third, and fourth diodes 34, 36, 38 and 40, respectively. An anode of the first diode 34 and a cathode of the second diode 36 communicate with a first terminal of the read element 32. An anode of the third diode 38 and a cathode of the fourth diode 40 communicate with a second terminal of the read element 32. A cathode of the first diode 34 communicates with an anode of the second diode 36. A cathode of the third diode 38 communicates with an anode of the fourth diode 40.
The first terminal of the read element 32, the anode of the first diode 34, and the cathode of the second diode 36 communicate with a first current source 42. The second terminal of the read element 32, the anode of the third diode 38, and the cathode of the fourth diode 40 communicate with a second current source 44. The first and second current sources 42 and 44, respectively, communicate with a supply potential 46. The cathode of the first diode 34, the anode of the second diode 36, the cathode of the third diode 38, and the anode of the fourth diode 40 communicate with a ground potential 48.
The ESD protection circuit 30 optionally includes fifth and sixth diodes 50 and 52, respectively. An anode of the fifth diode 50 and a cathode of the sixth diode 52 communicate with the first terminal of the read element 32 and the first current source 42. A cathode of the fifth diode 50 and an anode of the sixth diode 52 communicate with a second terminal of the read element 32 and the second current source 44.
The current sources 50 and 52, respectively, bias the read element 32 during normal operation. The diodes 34, 36, 38, 40, 50, and 52 form parallel back-to-back forward-biased diode shunting circuits. The diode shunting circuits limit a maximum voltage that is applied to the read element 32 to a forward biased turn-on voltage of one of the diodes 34, 36, 38, 40, 50, or 52. The maximum voltage of the shunting circuits is typically 0.7V for silicon junction diodes and 0.4-0.5V for Schottky diodes. GMR sensors begin to experience stress at 0.6-0.7V. Therefore, the range of protection offered by the diode turn-on voltage of conventional shunting devices is usually sufficient for GMR sensors.
However, tunneling giant magneto-resistive (TGMR) sensors are increasingly being used as read elements in magnetic storage systems. TGMR sensors have a very thin tunneling junction and begin to experience stress at approximately 0.3V. Therefore, the forward-biased turn-on voltage of either silicon junction diodes or Schottky diodes is not low enough to protect the TGMR sensor from ESD damage. Additionally, there are no conventional diodes that have a forward-biased turn-on voltage that is less than or equal to 0.3V.